std::isdigit is an STL function to test of a character is a decimal digit.


#include <cassert>

#include <string>



int main()

{

  const std::string s = "1a";



  //'a' is no decimal digit

  assert( std::isdigit(s[0]));

  assert(!std::isdigit(s[1]));



  //'a' is a hexadecimal digit

  assert( std::isxdigit(s[0]));

  assert( std::isxdigit(s[1]));

}

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(C++) std::isdigit